The present invention relates to a cubic boron nitride powder suitable for the subsequent production of cutting tools and a method for its production.
Cutting tools having cutting edges formed of a super hard abrasive such as a cubic boron nitride (cBN)-based material are manufactured by powder metallurgical techniques and are mainly used for the machining of cast iron and hardened steel. For cast iron, a tough material with 80-100 wt % cBN is used, while for hardened steel 10-50 wt % of TiC, TiN or Ti(C,N) is usually added. This addition decreases toughness, but greatly improves the chemical stability of the material. Most often, the PcBN (polycrystalline cubic Boron Nitride) material also contains smaller amounts (typically &lt;10 wt % each with a total maximum content of all such materials being 25 wt %) of other components, e.g., Co, Ni, WC, Al, AlN and Al.sub.2 O.sub.3. These are either added to the raw material powder or obtained during processing.
PcBN cutting tools are mainly produced in two different ways:
i) By high pressure/high temperature (HP/HT) sintering of a PcBN powder mixture into a solid body that is cut and ground into a finished cutting tool insert; or PA1 ii) By HP/HT-sintering a thin layer of PcBN powder which simultaneously bonds to a substrate (usually a cemented carbide disc), from which smaller pieces (chips) are cut out. These chips are brazed onto a regular carbide tool (e.g., insert, end-mill, drill) and ground to the finished state. The tools are relatively expensive to produce due to the many steps the product must undergo before it is finished. Also, usually only one or two cutting edges per tool are available. PA1 mixing PcBN or diamond with a liquid and an agent to form a homogenous slurry of a desired composition; PA1 forming spherical powder agglomerates of said mixture; PA1 filling the said agglomerates into recesses of a cemented carbide or cermet substrate; PA1 removing the agent from the said agglomerates at a suitable temperature and atmosphere; and PA1 sintering and simultaneously bonding the agglomerates to the substrate in a container under HP/HT-conditions to form a composite body.
Through U.S. Pat. No. 5,676,496, a technique is known for producing PcBN cutting tool inserts in a more cost efficient way. This is achieved by placing a cemented carbide or cermet substrate into a container and then packing PcBN powder into appropriately placed grooves in a substrate. The container is then HP/HT-sintered so that the PcBN powder is consolidated to a fully dense body, which is simultaneously bonded to the substrate. The substrate/PcBN compound may then directly be ground to a cutting tool insert.
Although the method described leads to extensive cost reductions per cutting edge, it has one major drawback in that the packing of a powder mixture containing PcBN into the grooves in principle must be done manually. The poor flow properties of PcBN powder in combination with the required groove geometry make automatic processing unreliable. Apart from obvious health hazards, manual packing may lead to uneven packing density and to excessive oxygen exposure of the PcBN powder. Uneven packing density makes it necessary to choose a larger groove dimension than desired to ensure that the amount of PcBN obtained is always sufficient. Careful control of the oxygen content in the PcBN powder is critical for the HP/HT sintering since excessive oxygen negatively affects the consolidation process. In principle, one would like to have a high and highly reproducible packing density and to minimize the oxygen pickup during handling and storage.